The present invention relates to a method of manufacturing a color conversion filter substrate and an organic EL display with excellent environmental resistance and productivity for displaying multi colors with high definition. More specifically, the present invention relates to a method of manufacturing a color conversion filter substrate and an organic EL display used for a display in an image sensor, personal computer, word processor, television, facsimile, audio equipment, video equipment, car navigation equipment, desk-top electronic calculator, telephone, mobile terminal equipment, industrial measuring equipment, and so on.
In recent years, the information technology has advanced greatly and the application of the technology has been expanding rapidly. In the field of a display device, there has been developed a high-definition display device with low power consumption and a high-speed response for meeting demands for mobility and displaying motion pictures.
An organic EL luminous device has excellent characteristics such as high contrast, low drive voltage, a wide view angle and a quick response time compared to liquid crystal display devices. Tang et. al reported that an organic EL luminous device formed of stacked thin films showed a high luminance of 1000 cd/m2 or higher at an applied voltage of 10 V. Since the report by Tang et. al, a great effort has been made to develop a plat panel display using an organic EL luminous device for a practical use. A green monochrome organic EL display has already been commercialized, and it is expected to develop a high-definition full-color display.
As another EL device other than a laminated layer of the organic molecules, attempts have also been made to develop an EL display device composed of an organic polymer material.
There have been three major approaches as a method of displaying multiple or full colors with the organic EL display. One of the methods has been disclosed in Japanese Patent Publication (Kokai) No. 57-167487, in which light emitting elements of the three primary colors (red, green, and blue) are arranged in a matrix form. In this method, it is necessary to arrange three types of light-emitting materials (R, G, and B) in a matrix form with high precision, thereby making it technically difficult to produce such a device at a low cost. Further, the three types of light-emitting materials have different life times, thereby shifting a color of the display with time.
As the second approach, Japanese Patent Publication (Kokai) No. 01-315988 has disclosed a method in which a color filter and a backlight emitting white light are used to display the three primary colors through the filter. However, it is difficult to obtain an organic light emitting device emitting bright white light for long time, which is necessary for obtaining bright three colors R, G, and B.
As the third approach, a method has been proposed in which fluorescent materials are arranged separately in a plane and absorb light from a light emitting device to emit light in multiple colors (Japanese Patent Publication (Kokai) No. 03-152897, etc.). This method for obtaining light in multiple colors from the fluorescent materials has been practically applied to a CRT, a plasma display, and so on. This method has an advantage in which a light emitting device with high brightness can be used as a light source. For example, a color conversion method in which blue light is converted to green light and red light has been proposed (Japanese Patent Publication (Kokai) No. 8-286033, etc.). It is possible to provide a display for displaying in full colors when a color-converting layer containing the fluorescent colorants is formed in a pattern with high resolution.
In order to display a color image with the methods described above, a display device driven by a thin film transistor (TFT) has been proposed recently. In such a device, it is difficult to provide a large opening in a system where light passes through a substrate having the TFT due to a shielding effect of a wiring portion. Therefore, there has been recently developed a top emission system in which the light radiates toward a side opposite to the substrate having the TFT.
In the top emission system, light emitting elements for the primary colors, i.e. red (R), green (G) and blue (B), are arranged separately in a matrix pattern. Therefore, it is necessary to precisely arrange light emitting materials for the RGB over the matrix pattern. Accordingly, it is difficult to manufacture such a system efficiently at a low cost. Moreover, the three light emitting materials have different luminescence characteristics and driving conditions. Therefore, it is still very difficult to obtain good color reproducibility for long time.
In a system where a backlight emits white light and a color filter is used to obtain the three primary colors, it is difficult to improve efficiency of the backlight.
In a color conversion system, fluorescent materials are arranged separately and absorb light so that the fluorescent elements emit fluorescent light in multiple colors. It is perceived that only the color conversion system can provide a high-resolution and bright organic EL display with the top emission method using the TFT drive system. Japanese Patent Publications (Kokai) No. 11-251059 and No. 2000-77191 have disclosed such color display devices.
In the case of an LCD (liquid crystal display) in which light needs to be transmitted from behind, it is necessary to make an area of the TFT, which blocks the light, as small as possible. On the other hand, in the case of an organic EL luminous device with the top emission method using the TFT drive system, the light-emitting device is located above the TFT. Therefore, it is possible to use the entire area of the substrate for forming the area of the TFT.
As a method of manufacturing such a top emission type multi-color organic EL display, there is a method in which a color conversion filter substrate is bonded to a TFT substrate on which an organic EL luminous device is formed. In this case, it is preferable to form a sealing space between the two substrates as small as possible. However, when a distance between the two substrates becomes too small, the two substrates may contact, resulting in damage of the organic EL luminous device. To resolve this problem, Japanese Patent Publication (Kokai) No. 11-297477 has disclosed an approach in which a supporting pillar as a spacer is formed on the substrate at a location other than the organic EL luminous device is formed.
As shown in FIG. 9, in a conventional color conversion filter substrate, a red color conversion filter layer 2, a green color conversion filter layer 3 and a blue color conversion filter layer 4 are arranged on a transparent supporting substrate. An overcoat layer 6 having gas barrier ability is disposed thereon as a flattening layer. The color conversion filter layers 2 (red), 3 (green) and 4 (blue) are each patterned independently in a rectangular shape. The overcoat layer 6 is formed with spin coating method or the like so as to cover the color conversion filter layers 2, 3 and 4.
It is necessary to form the overcoat layer having a thickness large enough to completely fill gaps between the color conversion layers and provide a sufficiently smooth surface. However, when the overcoat layer has too large thickness, a viewing angle is lowered. Therefore, it is important to form a thin overcoat layer while obtaining a sufficient degree of flatness.
When the overcoat layer is formed directly on the color conversion filter layers with the spin coating method or the like, the overcoat layer is formed in a very thin layer. However, due to a problem of coverage, the overcoat layer has an uneven surface with several μm undulations.
As shown in FIG. 8, in the color conversion type multi-color organic EL display with the top emission type, the color conversion filter substrate is bonded to the organic EL luminous device substrate facing with each other. It is preferable that a sealing distance between the two substrates be small, specifically 0.5 to 5 μm, similar to the overcoat layer, so that an optical path is short to prevent the viewing angle from lowering. However, there are undulations on the surface of the overcoat layer. Thus, it is difficult to maintain the sealing distance between the two substrates at a small, constant value using supporting pillars 11 as a spacer.
Moreover, when the supporting pillar is formed with the spin coating method and patterned with the photolithography method, the material utilization rate is very low as the supporting pillars are formed only in a very small area.
In view of the problems described above, it is an object of the present invention to provide a method of manufacturing a color conversion filter substrate in which a thin overcoat layer has sufficient surface smoothness. Further, the overcoat layer is provided with a projection as a supporting pillar, so that the color conversion filter substrate has a stable light emission characteristic over a long period of time.
Further objects and advantages of the invention will be apparent from the following description of the invention.